Hydraulic fluid composition



United States Patent 3,091,590 HYDRAULIC FLUID COMPOSITION Neal W. Furbyand Charles D. Newnan, Berkeley, Calif assignors' to California ResearchCorporation, San Francisco, Calif., a corporation of Delaware NoDrawing. Original application Aug. 28, 1953, Ser. No. 377,240. Dividedand this application May 20, 1957, Ser. No. 660,054

1 Claim. (Cl. 252-78) The present invention relates to the preparationof novel compositions useful as hydraulic fluids. More particularly, theinvention has to do with the preparation of such compositions based oncertain organo-silicon compounds as essential ingredients. Thisapplication is a division of application Serial No. 377,240, filedAugust 28, 1953, which in turn is a continuation-in-part of applicationSerial No. 215,638, filed March 14, 1951.

In general, a suitable hydraulic fluid is required to have certainproperties, such as good wear characteristics in the lubrication ofmoving parts of hydraulic systems; oxidation-corrosion stability; goodvolatility characteristics; compatibility with the seals employed in thehydraulic system; resistance to fire; good shear characteristics, thatis, retention of body and viscosity when subjected to the shearingaction of hydraulic pumps and when passing through small openings ororifices at high pressure; and most importantly, a goodviscosity-temperature relationship, that is, small change in viscosityover a wide temperature range, such as is encountered, for example, inthe operation of aircraft. Other additional requirements of a suitablehydraulic fluid are case of handling, non-toxicity, etc.

The fluids heretofore proposed as hydraulic fluids, while possessing oneor more of the aforesaid desired properties, are lacking in one or moreother properties or do not possess the desired properties to an extentregarded as satisfactory. Hydraulic fluids based on mineral oils, forexample, have heretofore been extensively used. Such fluids have certaindesirable characteristics, for example, good lubricating properties, butare deficient in other respects, for example, generally inferiorviscosity-temperature characteristics, etc.

We have discovered that hydraulic fluids based on a blend of alkylsilicone and alkyl silicate possess all the desirable attributesenumerated above, and have excellent temperature-viscositycharacteristics. Moreover, the fluids prepared in accordance with theinvention admit the in corporationof additives which impart to the finalfluid additional desirable properties or improve those already possessedby the blend. Compositions heretofore proposed as hydraulic fluids aregenerally not only inferior from a viscosity-temperature standpoint, butthose that may have a satisfactory viscosity-temperature relationshipare deficient in that they do not permit the incorporation of agents toimprove one or more of the properties of the fluid. In addition, we havefound that our fluids have superior volatility characteristics; that is,as compared with other fluids of like viscosity, our fluids are lessvolatile, as a result of which evaporation losses are reduced, as is theformation of heavy residues which adversely affect viscosity, lubricity,etc.

In brief, the fluid compositions of the present invention are made up ofa total of from about 85 to 97% by weight of tetra-alkyl silicate,together with from about 3 to 15% by weight of dialkyl silicone polymer.

The alkyl silicate component of the blend can be described as one havingthe general formula OR RO-Sl-OR ice where the Rs, which may be the .sameas or different from one another, represent alkyl groups of from 1 toabout 18 carbon atoms each. Preferably, however, each of the Rsrepresents a branched-chain alkyl group of from 5 to 8 carbon atoms, thebranched radicals herein referred to being those derived from secondaryor tertiary alcohols, or from primary alcohols wherein branching occursin the 2- position. Specific examples of tetraalkyl silicate esterscontemplated by the invention are tetraethyl silicate, tetra(Z-butyl)silicate, tetra(n-octyl) silicate, as well as those silicates having thepreferred structure indicated above, such as tetra(Z-methyl-lbutyl)silicate, tetra(2-methyl-2-butyl) silicate, tetra(nhexyl) silicate,tetra(2-hexyl) silicate, tetra(Z-ethyl-lbutyl) silicate, tetra(Z-heptyl)silicate, tetra(Z-octyl) sili cate, and tetra(Z-ethyl-l-hexyl) silicate.

The alkyl silicone polymer component of the blend of the presentinvention can be represented by the general formula,

where n is an integer having a value of at least one and wherein the R sare lower alkyl'radicals of from 1 to 5 carbon atoms each. Said siliconepolymer is one which has a viscosity of at least 1,000 cs., as measuredat 77 F., and which may have a viscosity as high as 10,000,000 or morecs. at 77 F. Preferably, the silicone component of the present blend isone having a viscosity between about 60,000 and 2,500,000 cs. at 77 F.Said silicone polymers may be of either branched or straight-chainconfiguration, and they may also incorporate various cross-linkages, allas is clearly understood in the silicone art, always provided that thesilicone polymerbe one which is soluble in the alkyl silicate componentof the blend to the extent of at least 3% by weight. Representativesilicone polymers are dimethyl silicone, diethyl silicone, methylethylsilicone, dipropyl silicone, dibutyl silicone and diamyl silicone.

The following examples illustrate compositions prepared in accordancewith the'invention.

Example 1 About 12 parts of dimethyl silicone having a viscosity ofabout 2,500,000 centistokes at77 F. was mixed with about 88 parts oftetra(Z-ethyl-l-butyl) silicate, the parts being by weight. Theresulting blend had the following viscosities:

Temperature, F.: Centistokes 6,097 198.1 147.6 210 81.9 400 30.5

Example 2 To about 91 parts of tetra( 2-ethyl-1-hexy) silicate there wasadded about 9 parts of diethyl silicone having a viscosity of about200,000 cs. at 100 F, the parts being 3 by weight. The resultingcomposition had an ASTM slope of 0.42 and the following viscosities:

Temperature, F.: Centistokes Example 3 Temperature, E: Centistokes 651,151 40 402 100 29.5 210 v 12.2 400 4.83

Ordinarily, it'is required that a hydraulic fluid suitable for aircraftuse have a minimum viscosity of centistrokes at 130 F., and a maximumviscosity of 500 centistokes at --40 F. The foregoing composition meetsall specifications for a satisfactory aircraft hydraulic fluid.

Example 4 A composition was made up of 8 parts of methylethyl siliconehaving a viscosity of 136,000 cs. at 77 F., together with 92 parts of amixture made up of tetra( 2ethylbutyl) silicate (75%) and tetra(Z-ethylhexyl) silicate 4 (25% The resulting composition had an ASTMslope of 0.436 and had the following viscosities:

Although the compositions described in the foregoing examples possessthose properties, such as anti-wear, viscosity, viscosity index,lubricity, thermal and oxidative stability, etc., to a degreesatisfactory to render them useful as hydraulic fluids meeting the moststringent requirements, it will be obvious to those skilled in the artthat additives, such as anti-wear and anti-rust agents, oxidation andcorrosion inhibitors, etc., may be incorporated in the blends to improveone or more properties thereof. Similarly, it will occur to thoseskilled in the art that the properties of the herein-describedcompositions are such as to render them useful in applications otherthan as power transmission fluids. For example, the fluids contemplatedby the invention may be used as lubricants for special purposes, such asin the lubrication of machine guns, aircraft instruments, etc.

We claim:

A power transmission fluid consisting essentially of to 97% by weight oftetra-octyl orthosilicate and 3 to 15% by weight of dimethyl siliconepolymer having a viscosity above 60,000 centistokes at 25 C.

References Cited in the file of this patent UNITED STATES PATENTS2,495,363 Barry et al. Jan. 24, 1950 2,681,313 Kather et al June 15,1954 2,684,336 Moreton July 20, 1954 2,746,926 Barry May 22, 1956

